The present invention relates to a fixing apparatus of an image forming apparatus, such as a printer, a facsimile machine or a copying machine, for fixing a toner image carried on a. sheet (plain paper, coat paper, an OHP (Over Head Projector) sheet, glossy paper, a cut sheet, such as a postcard or an envelope) to the surface of a sheet. More particularly, the present invention relates to a fixing apparatus having two pressing rollers which are pressed against one heat roller.
Hitherto, a fixing apparatus has been disclosed (refer to Japanese Patent Publication No. Sho. 58-21262) which is structured as shown in FIG. 8 in such a manner that two pressing rollers are sequentially pressed against one pressing roller beginning with a more upstream pressing roller in a direction of rotation of the one pressing roller.
The apparatus disclosed in Japanese Patent Publication No. Sho. 58-21262 has two pressing rollers, which are arranged in such a manner that a downstream pressing roller has a higher circumferential speed, or a pressing belt arranged between two rollers. Thus, a recording medium moved to the surface of the heat roller is brought into contact with the wide area of the two rollers or the pressing belt. As a result, toner can efficiently be softened at lower temperatures.
As described above, the conventional fixing apparatus shown in FIG. 8 has the structure that the circumferential speed of the pressing roller 3 is higher than that of the pressing roller 2. Therefore, a tension is applied to the sheet between the pressing rollers 2 and 3 after the leading end of the sheet has been allowed to pass through a pressing portion between the pressing roller 2 and the heat roller 1, and then allowed to pass through a pressing portion between the pressing roller 3 and the heat roller 1. As a result, the sheet can satisfactorily be brought into contact with the heat roller 1. That is, the tension is not applied to the sheet before the leading end of the sheet reaches the pressing portion between the pressing roller 3 and the heat roller 1. As a result, the sheet cannot satisfactorily be brought into contact with the heat roller 1.
That is, the conventional apparatus shown in FIG. 8 encounters considerably great difference in the heating value which is applied to toner on the sheet before and after the leading end reaches the pressing portion by the pressing roller heat roller 1. Thus, there arises a problem in that nonuniformity (nonuniformity in the strength of fixation, the color development characteristic, the transparency and the like) takes place in the leading end portion and the central portion of the sheet. Also the trailing end of the sheet has a similar problem. That is, the heating value, which is applied to toner on the sheet, becomes considerably different before and after period until the trailing end of the sheet passes through the pressing portion by the pressing roller 2 and the heat roller 1. As a result, results of the fixing operation have nonuniformity between the central portion of the sheet and the trailing end portion of same.
Although the above-mentioned structure attains an effect of softening the toner, the conventional structure has a problem in that a large quantity of stacked toner layers cannot satisfactorily strongly be fixed to the surface of a recording medium. Another problem arises in that a required result of the fixing operation by using heat cannot be obtained because the temperature of the recording medium is lowered during a period in which the wide areas of the recording medium and the heat roller are brought into contact with each other. What is worse, a slight difference in the speed between the heat roller and the pressing roller can result in a toner image, the surface of which is brought into contact with the rollers, being disordered.
A fixing apparatus has been known which is of a type having a heat roller and a pressing roller arranged to be pressed against the heat roller and structured in such a manner that a sheet having toner thereon is allowed to pass through the two rollers to heat, melt and fix toner to the surface of the sheet. There is apprehension that the fixing apparatus of the foregoing type encounters a problem that the sheet is wound around the heat roller because of the adhesive force of molten toner. If a color image is obtained by melting and fixing toner images in a plurality of colors stacked on a sheet, transparency must be realized by sufficiently heating, melting and mixing stacked toner in the plural colors. Therefore, the sheet can furthermore easily be wound around the heat roller.
On the other hand, an inventor of the present invention has found that the transparency of a toner image deteriorates if a large quantity of oil is applied to the surface of the heat roller. The reason for this will now be described. If oil in a large quantity is applied to the surface of the heat roller, the toner is heated and melted under the pressure through the large quantity of oil. As a result, the pressure of the surface of the heat roller is dispersed by the layer of the large quantity of oil, causing the smoothness of the surface of the heat roller not to be reflected on the surface of toner. Thus, it is conceivable that the resulting irregular reflection will deteriorate the transparency.